Nuclear magnetic resonance (NMR) techniques are widely used for analyzing properties of fluids and solids. NMR instruments may be placed into three classes: 1) magnetic resonance imaging (MRI) devices, which emphasize high spatial resolution; 2) NMR spectrometers, which have little or no spatial resolution but are able to resolve nuclear resonances that are closely-spaced in frequency; 3) time-domain NMR devices with neither spatial nor frequency resolution. In general, but recognizing there are many exceptions, MRI devices are used primarily in medicine and biological research to image live organisms, NMR spectrometers are used by chemists and biochemists to analyze structures of molecules, and time-domain NMR instruments are used to analyze materials properties at a coarser level than the molecular scale.
Nearly all modern NMR spectrometers are large (greater than 1 m3) and use superconducting magnets cooled to cryogenic temperatures. The magnet systems and associated NMR probes and electronics are expensive (roughly $300 K to $10 M) and they require substantial facilities and maintenance support. There are also some less-powerful NMR spectrometers that use ambient-temperature permanent magnets. These machines are similar in size to the smallest of the superconducting magnet systems, but are somewhat less expensive to purchase and maintain.
The power of NMR spectroscopy for chemical analysis derives from its ability to resolve small “chemical shifts” of the nuclear magnetic resonances due to differing chemical environments. For example, the magnetic resonance frequency of hydrogen nuclei (protons) in a CH3 chemical group within a liquid alkane differs from the resonance frequency of the protons in a CH2 group by about 0.5 parts-per-million (ppm). Nearly all chemical shifts of protons in organic molecules are in the range 0 to12 ppm. Chemical shift ranges are larger for some nuclear species and smaller for others. Because of the widespread application of proton NMR and the central role of hydrogen in organic chemistry, an NMR device is not usually considered to be a spectrometer unless it can resolve most proton chemical shifts. For our purposes, we define an NMR spectrometer to be an instrument with resolution better than 1.0 ppm for protons. Thus, time-domain NMR devices have spectral resolution poorer than 1.0 ppm and do not have spatial resolution.
In typical NMR spectrometers, a sample is placed in a region of uniform magnetic field. A wire coil and electronic circuits are provided to both apply radio-frequency (RF) pulses to the sample (transmit) and to detect RF signals from the sample (receive). In a typical experiment or measurement, the protons (or other nuclei under study) in the sample are first allowed to polarize in the magnetic field. Establishing equilibrium polarization takes a time which is known as T1 in the NMR art. This time ranges from a fraction of a second up to 20 seconds, depending on the composition of the liquid sample. Next, one or more RF pulses are applied to the sample with frequencies at or near the resonance frequency at which the protons freely precess in the applied uniform magnetic field. The transmit pulses have the effect of tilting the proton polarization relative to the direction of the applied field. After the transmit pulse is ended the protons precess and create a time-varying magnetic field in the coil. The time-varying field induces a signal voltage in the coil which may be amplified and recorded. Either immediately, or after a delay time, the receive signal is recorded and transferred to a computer for processing.
A particular sequence of transmit pulses, delay times and data acquisition periods, together with all of the parameters needed to precisely define the sequence (times, frequencies, phases, acquisition rates) is know in the NMR art collectively as a pulse sequence. The design of NMR pulse sequences to extract desired information from a sample is a well-developed area of knowledge. A great variety of pulse sequences are known in the art, ranging from simple ones with a single transmit pulse and a single data acquisition time to much more complex sequences with many transmit pulses, time delays, transmitter phase and amplitude adjustments, and data acquisition periods. The goals of these sequences vary widely, including enhancement of spectroscopic resolution, enhancement of signal-to-noise ratio, detection of spin-spin coupling between different groups of protons (or other nuclei), suppression of strong solvent signals, suppression of coupling between like or unlike nuclei, transfer of polarization from one group of nuclei to another, measurement of relaxation rates, measurement of diffusion within the fluid and measurement of fluid flow.
Permanent magnets can be, and often are, used to create the uniform magnetic field. Such permanent magnets typically used for this purpose are made from hard magnetic material with high remnant magnetization, such as neodymium iron boron or samarium cobalt. Permanent magnets of this type have large temperature coefficients of magnetization, typically in the range 300 to 2000 ppm/° C. Because the magnetic resonance frequencies are proportional to the applied magnetic field, this implies that permanent magnet NMR spectrometers have temperature coefficients of the resonance frequencies also in the range 300 to 2000 ppm/° C. To achieve spectral resolution below 1 ppm, this temperature coefficient has to be negated, stabilized, or compensated for in some way. Methods known in the art, as applied to larger NMR spectrometers, include use of a temperature compensating magnetic material within the magnet, temperature control of the magnet, and the field-frequency lock method.
The best magnetic uniformity of most typical permanent magnet assemblies used for NMR devices is at the geometrical center of the magnet assembly, so the main channel RF coil is typically placed at the geometrical center of the magnet assembly for best NMR signal resolution. Some magnetic field shimming can be used to increase uniformity of the magnetic field produced by the magnet assembly, if necessary or desired.
The foregoing examples of related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those skilled in the art upon a reading of the specification ad a study of the drawings.